Does a photon have a measurable lifespan, relative to itself? I mean, a photon that travels 1,000 light years has a lifespan of 1,000 years relative to us, the observer. But the photon traveled at c, so ... instantaneous/simultaneous beginning and end, relative to itself?
A photon is infinitely long-lived (it can't decay into anything), which is perfectly consistent with having time stopped.
But, we don't actually observe the singularity. Isn't observation limited to up to (but not quite including) the event horizon?
From one point of view, that could be turned into an argument that the event horizon is the entire structure of the black hole. The history of particles which have fallen into the hole seem to be contained in the topology of the surface.
Whether the singularity exists becomes a pragmatic choice: Thinking about black holes is a lot simpler if a particular interior structure is presumed, and, generally, scientists rather prefer to keep models as simple as possible.
It's a bit trickier than that. According to general relativity, if you are falling into a black hole, you don't notice that there's anything unusual about the horizon at all. So, other than tidal forces, which might or might not be noticeable outside the horizon (depending on the size of the black hole), you don't notice anything weird at all until you hit the singularity. This is part of the difficulty understanding Hawking radiation in black holes.
A question that I've had about light transmission is whether we could model this as discrete events, with the only physical events being the emission and absorption, with the absorption delayed according to the intervening distance, much as we would do if running a discrete event simulator, and sort-of what is done when rendering a scene: We compute the paths that light will follow, and render only the final point reached by light.
That's one way of thinking about quantum mechanics (whether of light or not), but you actually have to compute all possible paths between the two events.
Though, to be honest, that's not a very accurate picture. An interaction on that level is more like, "Particle-antiparticle pair pops out of the vacuum. Particle absorbs photon. Particle re-emits photon. Particle-antiparticle disappear back in to the vacuum." The photon is a single quantum of energy - it doesn't do anything by half-measures.
This is a common and pretty good way of explaining things. However, a more accurate way of explaining it in simple language has been put forward by Matthew Strassler (I recommend his blog very highly if you are interested in particle physics). Virtual particles aren't really particles. The idea of particle/wave duality has come up in this thread already. Virtual particles are better describe as packets of wave rather than particles, so it's more like a photon goes along, dissolves into a lump of electron wave, and then reforms. And it does this constantly.
Well, here'e the thing - by the physical laws as we currently know them, moving faster than light is... nonsense. It cannot happen. In order to accelerate anything with mass to the speed of light requires literally* infinite energy. Accelerating beyond the speed of light then requires *more* than infinite energy.
Just to add about something weird. Umbran is perfectly correct for anything with
real mass. It is perfectly mathematically consistent to have a particle with
imaginary mass (as in square root of -1) that can
only travel faster than light. These are called tachyons. However, in our current understanding of particle physics, tachyons are not stable particles but instead represent an instability, like sitting on the top of a hill is unstable to falling down the hill.
And even then, not *all* travel at FTL speeds ends up with you travelling backwards in time. What we can say is that, if you can travel faster than light, it is possible to arrange a path such that you return to your starting physical position before you left it. Not that *every* path results in this, but such paths do exist. Time travel becomes possible, not mandatory.
Spot on.
Something traveling faster than light would be going backwards in time in some frame of reference.
But anyways, what this would look like to the traveler, I don't know. Watching it all happen you would have something like:
The time traveler would always think that he/she is traveling forward in time, despite ending up in the past of when he/she left. It is a continuous process. That's the weird bit.
Scott DeWar said:
Ok, I am kinda picking up on what you are saying, but what if there is a general, all encompassing, here-to-fore unmeasured or even searched for, gravitational force pulling on the photons that prevent the photons from going faster. As for photons, I have to ask, that, is it because they are massless that allows them to be simultaneously created and at c at that same instant?
All I can say is that the mathematics work perfectly if there are no forces acting on the photons. You're precisely right that it is their masslessness that makes them move at speed c at all times, including as soon as they are created.
Morrus said:
It's something I see a lot. Folks say "oh, well, if x is y, I posit z!" (often in a "my logic trumps centuries of scientific theory!" kinda way) when what they're arguing against is a sentence they read somewhere which is a crappy rough attempt to translate 90 pages of equations into a simple English concept in one sentence. If there's a flaw, it's likely in that sentence; the 90 pages of equations may well have a flaw, but it won't be deduced from that English sentence - all that will be deduced is that a sentence is a crappy way to communicate 90 pages of equations.
Morrus, I can't tell you how much I, as a physicist, appreciate that you understand this and wrote it in this thread. I (and many many physicists) deal with this kind of logic all the time, and it is sometimes very difficult to get people to understand that mathematics is required. It's the same thing with the new paper by Hawking that has been a big deal in the press recently. It's a nice idea (which is actually very similar in some ways to what other people have done), but it's right now impossible to evaluate because there is no math (though other people's similar ideas are fleshed out mathematically). Physics can
only be properly understood with a grasp of the mathematics involved. That's why Newton had to invent (or co-invent) calculus to describe planetary motion. I think it's possible to get an idea of what's happening with an explanation in words, but a full understanding and the ability to do physics simply requires a lot of math.
